Electrostatic chucks (ESCs) are gaining popularity in the semiconductor wafer processing industry because they offer a variety of advantages over mechanical or vacuum chucks. Unlike vacuum chucks, electrostatic chucks are ideally suited for use in vacuum environments. As compared with mechanical chucks, ESCs have fewer moving parts, eliminate front side coverage of the wafer, and lessen risk of particle contamination. Ideally, ESCs offer the potential for rapid and strong clamping and declamping of semiconductor wafers over a range of temperatures without bowing or warping the wafer.
Although ESCs can have multiple structural configurations, they generally require a surface dielectric layer covering an electrode. A variety of dielectric coatings have been proposed for use in ESCs, including alumina (see, e.g., U.S. Pat. Nos. 5,413,360 to Atari et al., 5,384,681 to Kitabayashi and 5,207,437 to Barnes et al.), silica (see, e.g., Barnes et al.), silicon nitride (see, e.g., Atari et al.), tantalum oxide, diamond (see, e.g., U.S. Pat. No. 5,166,856 to Liporace), polyimide and polytetrafluoroethylene.
One of the challenges in ESC manufacture is the ability to reproducibly prepare dielectric coatings on metal electrodes that are durable and perform well over a range of temperatures and electric fields. Some glass or glass-ceramic compositions are known for making coatings on metal surfaces; however, the dielectric properties of these coatings and/or their compatibility in ESCs are not disclosed.
For example, U.S. Pat. No. 4,689,271 to Schittenhelm et al. discloses steel substrates coated with electrically insulating coatings of a fusible material comprising about 43-47 wt. % B.sub.2 O.sub.3, 29-33 wt. % CaO, 10-15 wt. % SiO.sub.2, 7-10 wt. % Al.sub.2 O.sub.3 and 1-2 wt. % MgO.
JP 60-103190 discloses borosilicate coated metal articles that are particularly suited for use as electronic components. Table 1 of this reference summarizes four examples in which aluminum substrates were coated with varying amounts of SiO.sub.2 (5-53%), B.sub.2 O.sub.3 (10.5-19%), Na.sub.2 O (0.0-4.2%), Al.sub.2 O.sub.3 (0.0-15.0%), ZnO (1.5-20%), BaO (0.0-18%) and/or other oxides. Stainless steel is another substrate disclosed.
U.S. Pat. No. 5,515,521 to Fu et al. discloses borosilicate glass compositions suitable as protective coatings for metal substrates, such as non-pickled steel substrates. The compositions according to Fu et al. comprise 49.1-56.2% SiO.sub.2 and 6.6-14.8% B.sub.2 O.sub.3. Dielectric properties of the coating are not disclosed.
U.S. Pat. No. 3,175,937 to Bayer et al. discloses a method of bonding metal parts with a glass adhesive, such as a glass adhesive comprising about 38 parts SiO.sub.2, 57 parts B.sub.2 O.sub.3, 5 parts Na.sub.2 O and 2 parts Fe.sub.2 O.sub.3. Combinations of glass and metal are selected so that the glass has a coefficient of expansion less than, or not appreciably greater than, that of the metal.
U.S. Pat. No. 3,379,942 to Davis discloses barium-borate glass compositions suitable for use as capacitor dielectrics that comprise stacks of interleaved metal and glass layers. The barium-borate glasses include about 25-35 wt. % BaO, 5-10 wt. % SiO.sub.2 and 1-6 wt. % Al.sub.2 O.sub.3. Davis teaches that the composition must comprise some amount of TiO.sub.2 because it improves chemical durability and maintains a low viscosity.
U.S. Pat. No. 4,221,824 to Leonard et al. discloses a method for enameling ferrous objects, such as steel, with a dielectric borosilicate coating including about 16-45 wt. % SiO.sub.2, 10-26 wt. % B.sub.2 O.sub.3 and 2-20 wt. % BaO and/or CaO. Leonard et al. teaches that the coating must comprise at least 5 wt. % of an adhesion promoting oxide, of which at least 1 wt. % must be cupric oxide.
The preparation of dielectric surface coatings for ESCs is complicated by factors including: stresses and porosity that are likely to form in coatings prepared from glass or ceramic green sheets (see, for example, Bang and Lu, "Constrained-Film Sintering of a Borosilicate Glass: In Situ Measurement of Film Stresses," J. Am. Ceram. Soc., 78(3), pgs. 813-815, 1995; Bang and Lu, "Densification Kinetics of Glass Films Constrained on Rigid Substrates," J. Mater. Res., 10(5), pgs. 1321-1326, 1995; and Choe et al., "Constrained-film Sintering of a Gold Circuit Paste," J. Mater. Res., 10(4), pgs. 986-994, 1995), and mismatch of thermal expansion coefficients (or linear coefficients of expansion). Unlike free films, and to a lesser extent sandwiched films, the exposed dielectric films on ESCs experience stresses that may lead to cracking or peeling of the dielectric layer. Further hazards for the ESC are exposure to reactive gases during processing of the semiconductor wafers, and chemical reaction with the silicon wafer. In addition, problems may arise from crystal growth or phase changes that may occur both in firing the coating on the ESC electrode as well as during temperature cycling during use of the ESC. Similarly, repeated electric field cycling during ESC clamping and declamping operations may degrade the coating. It has been reported that grain boundaries are critical sites at the clamping surface of the ESC. See Watanabe et al., "Electrostatic Charge Distribution in the Dielectric Layer of Alumina Electrostatic Chuck," J. Mater. Science 29, pgs. 3510-3516, 1994. Thus, the type and distribution of grain boundaries in the dielectric coating are another important consideration in the design of the ESC coatings. In summary, the myriad of complex and interrelated factors make the task of providing dielectric coatings for ESCs both challenging and unpredictable.